The present invention relates to a power-assisted steering system for applying an auxiliary steering force to a steering mechanism of a vehicle by an electric motor, and more particularly, to the detection of current flowing to the electric motor.
Generally, an electric power-assisted steering system has been in use in which a steering mechanism is provided with an auxiliary steering force by driving an electric motor in accordance with a steering torque exerted on a steering wheel by a driver of the vehicle. A torque sensor is provided in the electric power-assisted steering system for detecting a steering torque exerted on the steering wheel, which is a manipulating device for steering the vehicle. A target value for a current (hereinafter, referred to as a “current target value”) that is to be supplied to the electric motor is set based on the steering torque detected by the torque sensor. Then, a command value that is to be provided to a driving device for the electric motor is generated through a proportional integration operation based on a deviation between the target current value and the value of current that is actually made to flow to the electric motor. The electric motor driving device includes a PWM signal generating circuit for generating a pulse width modulation signal (hereinafter, referred to as a “PWM signal”) having a duty ratio which corresponds to the command value and a motor driving circuit which is configured by utilizing a power transistor which is on and off in accordance with the duty ratio of the PWM signal and applies to the electric motor a voltage which corresponds to the duty ratio. A current caused to flow to the electric motor by the application of the voltage is detected by a current detector, and a difference between the current target value and the current detected value is used as the deviation for generating the command value.
In the configuration, there occurs a case where current is actually detected by the current detector irrespective of the fact that current that is made to flow to the motor is nil. The current is referred to as an offset current, and a detected current value by the current detector during the operation of the motor is such that the offset current is superimposed on the current flowing to the motor. This offset current constitutes a cause of the generation of an error (referred to as an “offset error”) in the detected current value detected by the current detector. In the event that the offset error is generated, since the detected current value does not coincide with the value of current actually flowing to the motors the current control of the motor cannot be implemented as set, and as a result, the driver is forced to be provided with a feeling of physical disorder.
Incidentally, while motors with a brush have been in use as a driving source of conventional electric power-assisted steering systems, in recent years, brushless motors are also in use in conventional electric power-assisted steering systems with a view to enhancing reliability and durability, and reducing the inertia. In the electric power-assisted steering systems utilizing the brushless motors, in case the off set error is generated in the power-assisted steering systems, a torque ripple is generated at the time of steering. Since this torque ripple causes the driver to feel a feeling of physical disorder much when manipulating the steering wheel, the offset error has been causing a specific problem in the electric power-assisted steering systems utilizing the brushless motors.
To cope with the problem, a patent literature No. 1 discloses a current detecting circuit for an electric motor in which current to the electric motor which connects so as to bridge the bridge circuits of switching elements is detected by a current detector, comprising a means for making a current detection signal of the current detector resulting when the electric motor is not driven correspond to a state in which current to the electric motor is nil. According to the current detecting circuit for an electric motor, the offset error can be corrected, and in the event that the correction of the offset error is performed in the electric power-assisted steering system, a torque ripple attributed to the offset error can be suppressed. Then, there has been proposed a conventional electric power-assisted steering system in which a detected value detected by a current detector at the time of assembly of the electric power-assisted steering system or activating the same (when a power supply is made by switching on an ignition) is written to be retained as an offset correction value, so that a detected value detected by the current detector during the operation of the electric motor is corrected by the offset correction value so retained (for example, refer to a paragraph entitled “Description of the Related Art” of a patent literature No. 2).
However, since the offset value of the current detector varies due to a change in environment such as a change in temperature during the operation of the motor after the power supply to the electric power-assisted steering system has been made, there occurs a case where the detected value of the current detector cannot be corrected sufficiently with the retained offset correction value.
To cope with this, there is also known an electric power-assisted steering system in which a special state such that no motor current flows during the control of the motor after a power supply has been made is set on the premise the aforesaid configuration exists, so that the offset correction value is updated with a detected value detected by the current detector in the state so set (for example, refer to the patent literature No. 2).
However, due to the necessity of setting the system in such a special state, there occur a case where the motor control operation discontinues to thereby make the driver feel a feeling of physical disorder and a case where timings when the offset correction value can be updated are limited, whereby the offset correction value cannot be updated when such a correction is really needed.                Patent Literature No. 1: Japanese Examined Patent Publication No. 2847406        Patent Literature No. 2: JP-A-8-119132        
With a view to solving the problems inherent in the related art, it is preferred that the offset correction value for use in correcting the offset error can be updated any time during the operation of the electric power-assisted steering system after the power supply thereto has been made.